In many communication applications full duplex transmit/receive (TX/RX) operation is required. One such application is in the implementation of the CDMA communication standard. A more specific application is in the area of wireless handsets and PCM/CIA cards in computers, which implement the CDMA standard.
Often one desires to implement a multi-band communication system. A typical example is a dual band wireless handset where the two bands implemented are the U.S. cellular (typically 800-1000 MHz) and U.S. PCS (typically 1800-2000 MHz). A current radio frequency (RF) realization of such architecture employs entire, separate TX and RX chains for the RF. This realization is driven by the rather wide separation of the two bands along with the fact that most of the components used are by necessity narrow band devices. Specifically, these narrow band devices include, but are not limited to, band pass filters (BPF's), power amplifiers (PA's) and duplexers used for either band.
Basic building blocks of present day dual band wireless CDMA handsets are BPF's and duplexers based on electromagnetic (em) resonators. Examples are short circuit monoblock and stripline filters, which support transverse electromagnetic (TEM) waves. They have an added advantage in that they're physically short as well as being relatively simple structures to fabricate. Others are surface acoustic wave (SAW) or film bulk acoustic resonator (FBAR) technologies. The relative merits and limitations of SAW and FBAR devices are well known to those skilled in the art. In all cases (TEM or acoustic resonator) the resultant filters or duplexers must meet tight specifications in their designated bands. Going to multiple bands conventionally requires using multiplexers in place of duplexers or multiple duplexers with multiplexers following them or the use of switches to select desired bands. Any of these options take up precious board space, add cost and complexity to the system.
The advantages of using ferro-electric (f-e) based components to design and fabricate low order, high performance filters have been outlined in U.S. patent application Ser. Nos. 09/904,631; 09/912,753; and 09/927,732. F-E tuning is limited, however, to a relatively narrow band around a desired operating frequency if the lowest possible insertion loss (I.L.) is desired. The requirement for minimum I.L. in wireless CDMA systems has also been discussed previously and will not be repeated here. F-e tuning as it applies in this application will be directed to its use with em resonators, such as, for example, monoblock, stripline and coaxial resonator-based devices. As is known to those skilled in the art, lumped elements (discrete inductors and capacitors) can be used at appropriate lower frequencies or at higher frequences where the added loss inherent to lumped element components can be tolerated.
The need to use two distinct RF architectures to realize two distinct bands is unfortunate, as it significantly increases cost and pwb board area required to realize the design. The need to specify and carry two sets of components further adds to cost and overhead. Clearly, if one wanted to realize say three, or more bands, the problems of increased cost, size and signal interference would make the attempted realization prohibitive, especially in high volume.
It would be advantageous if one could design a dual band (or higher) system with one set of components rather than using two (or more) sets.